A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning” -direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
Modern high quality optical systems, including those used in lithography apparatus, may use one or more deformable optical elements to correct for aberrations that appear during use, such as those caused by lens heating. Where the deformable element is a transmissive lens element, adjustment of the lens shape may be achieved by manipulation of a peripheral edge of the lens element (so as not to interfere with the path of radiation through the optical system) using out-of-plane forces (perpendicular to the plane of the deformable lens element in question). Aberration levels can be improved using such methods, but only to a limited extent. This is because the degree of precision with which a generalized distortion can be corrected using only out-of-plane forces applied around the edge of the lens element is limited. Better compensation may be achieved using reflective elements where corrective forces can be applied all over the rear side of the element without interfering with the operation of the element. However, it is not always possible to replace all transmissive elements with reflective alternatives.